Magnetization of permanent magnets is often the final step in the manufacturing process, and produces the required arrangement or pattern of poles in the product. This pattern can be as simple as a single pair of poles at either end of a bar or rod shaped magnet, or the pattern can be relatively complex, such as skewed, multi-pole patterns on the external surface of a hollow cylinder. The latter type of magnetization pattern is sometimes used in electric motor applications.
There are several ways of "imprinting" the required arrangement of poles, depending upon the complexity of the magnet, and the materials from which the magnet is made. The simplest method is to utilize the field from another permanent magnet, but this can only be used where the part can be magnetized in a low applied field. The next level is to use an electromagnet, either as a solenoid coil, or as part of a C-core magnetic circuit. For materials that are difficult to magnetize (high coercivity), the solenoid may need to be powered by a very high current discharge from a bank of capacitors.
These methods result in very simple magnetization patterns, typically with a north and a south pole at each end or face of the magnet, with the direction of magnetization being described by a single vector. For more complex patterns, such as the idealized eight pole configuration shown in FIG. 1, two other types of magnetization fixtures could be considered. The two include the steel cored fixture shown in FIG. 2, and the serpentine wound fixture shown in FIG. 3a. FIG. 3b is a cross sectional view of a hollow cylindrical magnet surrounded by the serpentine wound fixture of FIG. 3a. Because the field pattern that these two fixtures create is not uniform, as is the case in an ideal solenoid, the resulting magnetization of the part is complex and will tend to be represented by the pattern shown in FIG. 4. The resulting poles can no longer be described by a single vector, as in the ideal case, and the resulting field pattern above the surface of the magnet will also be different. Because of the focusing effect of the non-ideal pattern, the peak strength of the field above the center of each pole will be higher than in the ideal case, and the zero crossing slope will be shallower. Depending upon the application requirements, this result may or may not be of benefit.
This type of multi-pole focused magnetization pattern has been widely reported and used to benefit in many cases. However, the invention described herein utilizes some of the features of focused magnetization and applies them in a novel way, to enhance the performance of certain magnet geometries that are typically used in sensor applications.
Lee et al., in U.S. Pat. No. 5,659,280, disclose a system for forming desired magnetization patterns in permanent magnet structures such as magnetic brush cylinder cores, utilizing a fixture having a plurality of magnetizing members selectively orientable to the permanent magnet structures and couplable to a capacitor discharge magnetization apparatus. Magnetic tip members having different flux focusing end configurations are employed to form corresponding polarization patterns in the permanent magnet cylinder structures.